1. Field Invention
This invention relates generally to producing VO.sub.2 powders and more particularly to a novel evaporative decomposition technique for producing such powders.
2. Description of the Prior Art
It is generally well known that solid VO.sub.2, unlike other vanadium oxides V.sub.2 O.sub.3, V.sub.2 O5, etc., exhibits a low temperature (about 67.degree. C.) phase transition from a monoclinic, semiconducting phase to a tetragonal metallic phase. During this phase transition, an abrupt change in certain physical properties such as electrical resistivity, magnetic susceptibility and infrared transmittance occurs. Because of its conveniently low transition temperature and, particularly, the large drop in near infrared transmittance, VO.sub.2 's thermochromic properties render it a popular candidate for use in thermal relays, electrical and optical switches, optical storage media, modulation devices, etc.
Also, it is well known that doping VO.sub.2 with other metal oxides can lower the transition temperature closer to room temperature thus enabling solid VO.sub.2 to become useful in additional applications.
The production of VO.sub.2 devices, and particularly doped-VO.sub.2 devices, in the form of solid films by techniques such as reactive sputtering and sintering has become quite advanced. However, a need has arisen to produce VO.sub.2 and doped VO.sub.2 in the form of high quality micron sized powders, particularly in various aerospace applications where thin films and/or epitaxially grown crystals cannot be employed. Production of such powders has met with limited, if any, success.
For example, phase transition studies on tungsten dioxide (WO.sub.2), doped VO.sub.2 powders were conducted as early as 1969 by Israelsson and Kihlborg, "The phase relations in the VO.sub.2 --WO.sub.2 system" Materials Research Bulletin 5, 19-30 (1970). The VO.sub.2 --WO.sub.2 powder was produced by initially preparing V.sub.2 O.sub.5. The V.sub.2 O.sub.5 was then chemically reduced to V.sub.2 O.sub.3 by hydrogen reduction for seven hours at 800.degree. C. Mixtures of V.sub.2 O.sub.3 and V.sub.2 O.sub.5 were heated in evacuated silica tubes, keeping the temperature at 1100.degree. C. for ten to twelve days, to produce VO.sub.2. In a separate reaction, H.sub.2 WO.sub.4 was heated to prepare WO.sub.3, which was thereafter chemically reduced in hydrogen to WO.sub.2. Finally, the WO.sub.2 was heated together with the V.sub.2 O.sub.3 and V.sub.2 O.sub.5 at 1100.degree. C. in order to produce the VO.sub.2 --WO.sub.2 mixed powders. However, particle size and product quality were too inconsistent for commercial applications.
In 1971, Rao, Natarajan, Subba Rao, and Loehman disclosed solid solutions of doped VO.sub.2 in "Phase Transitions and Conductivity Anomalies In Solid Solutions of VO.sub.2 and TiO.sub.2, NbO.sub.2 and MoO.sub.2 Journal of Physical Chemistry Solids, 32, 1147-1150 (1971). However, the process required melting stoichiometric proportions of precursor solid oxides in an arc furnace. Polycrystalline pellets are produced but were not commercially useful.
Neither of these efforts produced homogeneous high purity unagglomerated micron sized powders. But, rather, only large pellets and/or highly agglomerated powders with inconsistent physical properties could be obtained. These techniques require such high temperatures that difficulty arises in achieving uniform product and consistent yields.
More recently, evaporative decomposition (hereinafter referred to as "spray pyrolysis") has offered a more promising technique for producing high quality mixed oxide powders. Fine powders of metal oxides have been produced by spray pyrolysis of metal salts such as nitrates, acetates, methoxides and formates. See, for example, T. J. Gardner and G. L. Messing, "Preparation of MgO Powder by Evaporative Decomposition of Solutions", Ceramic Bull. 63 [12] 1498-1504 (1984); M. Ramamurthi and K. H. Leong, "Generation of Monodisperse Metallic, Metal Oxide and Carbon Aerosols", J. Aerosol Sci. 18 [2] 175-191 (1987); A. Clearfield, A. M. Gadalla, W. H. Marlow and T. W. Liningston, "Synthesis of Ultrafine Grain Ferrites", J. Am. Ceram. Soc. 72 [10] 1789-92 (1989); W. R. Moser and J. D. Lennihoff, "A New High Temperature Aerosol Decomposition Process for the Synthesis of Mixed Metal Oxides for Ceramics and Catalysts and their Characterization", Chem Eng. Comm. 83, 241-259 (1989); and "T. P. O'Holleran, R. R. Neurgaonkar, D. M. Roy and R. Roy, "EDS for the Preparation of .alpha.-Fe.sub.2 O.sub.3 ", Ceram. Bull. 57 [4] 459-460 (1978). However, the only vanadium oxide to be produced from spray pyrolysis has been V.sub.2 O.sub.3.
V.sub.2 O.sub.3 powder was produced by Sullivan in 1990 as reported in "V.sub.2 O.sub.3 powder by Vaporative Decomposition of Solutions and H.sub.2 Reduction", J Am Ceram Soc, 73 (12), 3715-3717 (1990). In this process, V.sub.2 O.sub.5 was dissolved in nitric acid for spray pyrolysis of a V.sub.2 O.sub.5 powder which was then reduced in hydrogen to V.sub.2 O.sub.3 powder. Since V.sub.2 O.sub.3 lacks the thermochromic properties exhibited by VO.sub.2 powders, spray pyrolysis of such nitrate solutions offers little incentive for solving the commercial problem.
Spray pyrolysis of sulfate solutions would appear to be a potential alternative since sulfate solutions have been evaporatively decomposed with limited success, in making magnesium, copper and iron oxide powders. See, for example, T. J. Gardner and G. L. Messing, "Preparation of MgO Powder by Evaporative Decomposition of Solutions", Ceramic Bull. 63 [12] 1498-1504 (1984); C. Roth and R. Kobrich, "Production of Hollow Spheres", J. Aerosol Sci. 19 [7] 939-942 (1988); and M. Ramamurthi and K. H. Leong, "Generation of Monodisperse Metallic, Metal Oxide and Carbon Aerosols", J. Aerosol Sci. 18 [2] 175-191 (1987). Evaporative decompositional studies help to predict what would occur during spray pyrolysis processes.
However, spray pyrolysis of sulfate solutions to produce VO.sub.2 powders, until now, has not been attempted. This is perhaps owed to the fact that evaporative decomposition studies, including thermal gravimetric analysis and chemical equilibrium evaluations at realistic O.sub.2 partial pressures, would predict that the thermal decomposition product of vanadyl sulfate (VOSO.sub.4) in H.sub.2 is V.sub.2 O.sub.3, and in N.sub.2 is a mixture of V.sub.6 O.sub.13 and V.sub.2 O.sub.5. In the noxious atmosphere of sulfur dioxide (SO.sub.2) (which produces unacceptable amounts of H.sub.2 SO.sub.4 and unacceptable amounts of V.sub.6 O.sub.13 in the presence of H.sub.2 O) VOSO.sub.4 has been thermally decomposed into residual amounts of V.sub.2 O.sub.4. See J. Tudo, "Sur L'Etude du Sulfate de Vanadyle et de sa Reduction par L'Hydrogene Sulfure: les Sulfures de Vanadium (Vanadyl Sulfate and its Reduction by Hydrogen Sulfide: Vanadium Sulfides)", Rev. Chim. Minerale 2 [1] 53-117 (1965). However, in the more acceptable atmosphere of N.sub.2, V.sub.2 O.sub.5 is produced, and as previously discussed, V.sub.2 O.sub.5 would be expected to reduce to V.sub.2 O.sub.3 even if the atmosphere was modified by the presence of H.sub.2 ; thus, rendering little incentive to try commercial production of VO.sub.2 powders by a spray pyrolysis process of sulfates.
Accordingly, a method for producing high quality unagglomerated micron-sized VO.sub.2 or doped-VO.sub.2 powder, without undesirable decomposition by-products, would be a novel and unexpected advancement in the art, and fulfill a long felt need in the industry. It would be particularly unexpected to produce such powders from spray pyrolysis of a sulfate solution in H.sub.2 atmosphere.
Therefore, it is a principle object of the present invention to provide a high quality VO.sub.2 powder.
It is another object to provide a method for producing the high quality VO.sub.2 powder at lower temperatures and without intermediate decomposition by-products.
It is another object to provide high quality doped-VO.sub.2 powders having room temperature transition by a method simple to implement and requiring a minimum of decomposition variables.
Other objects, advantages, and novel features of the invention will become apparent from the following summary and detailed description of the invention.